6.47).
A point worth remembering is when the modulation is raised to a power of say K, then the phase
change has increased by the same amount. This phase amplification should be removed when estimating
absolute terms. Hence the error signal can be derived as
(6.122)
The interested reader should consult [38] for alternative forms of frequency error discrimination
techniques, such as ???Differentiator AFC??? and ???Cross Product AFC,??? respectively.
e
1
8
# Im g5[I(t) jQ(t)]86
x(t) cos [8u(t) 8vt] j sin [8u(t) 8vt]
x(t) [I(t) jQ(t)]8 [ej(u(t)vt)]8
RECEIVER DIGITAL SIGNAL PROCESSING 335
(a)
(b)
Quantizer
Img Error Signal
I2 + jQ2 I1 + jQ1 I1 ??“ jQ1
( )*
{ } X
Quantizer
Error Signal
Out
In
Img
I2 + jQ2 I1 + jQ1
FIGURE 6.46 Alternative forms of the AFC-Two phase discriminator.
6.6.3 BER Performance Degradation
In this section, we will present the BER performance of the 1NEC, 2NEC, MLDD, and DD detectors
previously described for various residual frequency offsets. In Fig. 6.48 we start with the performance
of the 2NEC as the frequency offset is varied from 200 to 800 Hz. The results clearly show as the
residual frequency offset increases performance degradation increases.
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